1. Field of the Invention
The present invention relates to a fixing apparatus for heating and fixing an unfixed image on a recording member, which apparatus is used in an image forming apparatus, such as a copier or printer.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic process, electrostatic recording process or the like, an image formed on a recording member is heated and fixed. A fixing apparatus used in such image forming apparatus uses a heating member which is kept at a predetermined fixing temperature, as is typically done in a heat roller method.
When the temperature of this heating member is increased to a fixing temperature, even if the supply of electric current to the heating member is stopped at the fixing temperature, the temperature continues to increase beyond the fixing temperature due to an inertia of temperature increase, i.e., a so-called overshoot, occurs. Thus, electric current being supplied is stopped at a temperature detected by a temperature detection member lower than the fixing temperature to allow for an overshoot amount, so that the temperature is kept constant.
Changes in temperature in a case where such control is performed are shown in FIG. 8. Concerning the temperature graph of FIG. 8, a temperature detection member for detecting the temperature of the surface of a heat roller is disposed outside a recording member transport area.
Reference letter a denotes the roller temperature while the fixing apparatus is in a stand-by mode. The supply of electric current is started or stopped at a temperature b' which is lower than the predetermined temperature b of a temperature detection section so that the above control of the temperature is performed.
Since the temperature in the central portion of the heat roller is highest, while it is lower towards both end portions thereof, the gradient of temperature increase during heating is greatest at the central portion and is lowest at the periphery of a portion where the temperature is detected by a thermistor. Therefore, the .DELTA.T exceeding the prescribed temperature C of the temperature in the central portion is larger than the .DELTA.T exceeding the prescribed temperature b of the temperature of the portions where the temperature is detected. As a result, even if the temperature control is switched at a temperature b' as shown and the overshoot is suppressed in the thermistor section, it is difficult to make the .DELTA.T zero for the central portion of the heat roller. If, on the contrary, such control is performed so that the .DELTA.T becomes zero for the central portion of the heat roller, a problem arises in that the temperature of the roller decreases near its end portions of the roller.
If the overshoot amount .DELTA.T is large, the image fixing temperature of a first sheet becomes high, and a high-temperature offset arises during the initial period of the fixing, particularly when the first image is being fixed. For this reason, it is desirable to suppress the .DELTA.T so that it is as small as possible.
Therefore, it has been suggested to decrease the supplying of electric current to the heat roller as the temperature becomes high so that the temperature increase gradient is lessened. Although in this method the temperature increase gradient is lessened and the overshoot amount .DELTA.T is decreased, thereby reducing the influence of high temperatures upon the image, the heat roller takes a longer time to reach the fixing temperature b by the lessened gradient of the temperature increase.